1. Field of the Invention
The present invention relates to an apparatus and method for continuous casting of steel including the step of induction-heating a molten steel surface in a mold and producing cast products having improved surface characteristics.
2. Description of the Related Art
In general, the surface characteristics of cast products obtained by continuous casting of steel are strongly dependent upon the condition and manner in which molten steel begins to solidify in the mold, that is, the conditions of the initial solidification.
The conditions of initial solidification are determined by a variety of factors such as (1) vibration (if any) of the mold; (2) friction (lubrication) of the mold and the cast products; (3) loss or escape of heat conditions in the vicinity of the meniscus on the molten steel surface; (4) flow characteristics of the molten steel in the mold, and others.
The initial solidification conditions are actually determined by many factors that influence each other in a complicated manner. Above all, it is believed to be important to provide and achieve special control of the thermal conditions existing at the meniscus in order to obtain cast products having good surface characteristics.
In order to vary the applicable thermal conditions, various methods are available such as varying the rate of heat escape by using various mold materials, and by heating the meniscus from outside the mold.
As disclosed in Japanese Patent Publication No. 57-21408, in a conventional mold used for continuous casting, an induction heating coil is arranged at the rear of a coiling plate of a mold made of copper. Since copper has high electrical conductivity, it is necessary in order to effectively heat the molten steel either to provide a low frequency to the induction heating coil, or, if a high frequency is applied, the thickness of the copper plate must be reduced as much as possible to approximately 1 mm, for example.
However, if low frequency is applied in such a case, the molten steel in the mold is stirred so as to become contaminated with mold powder, impairing the quality of the product.
If the thickness of the copper plate is further reduced, the copper plate is vulnerable to damage by heating, with the serious result that when the molten steel is brought into contact with cooling water in the mold, a steam explosion is likely to occur.
Variation of thermal conditions can be achieved by changing the mold material, including the use of a Ni--Cr--Fe alloy having low heat conductivity and high strength at a high temperature, as disclosed in Japanese Patent Laid-Open No. 3-264143.
However, a serious drawback of this approach is that the thermal conditions at the meniscus cannot then be controlled with precision or accuracy. For example, the thermal conditions at the meniscus are at least partially dependent upon the casting conditions, such as the casting speed and the temperature of the molten steel introduced into the mold, causing ineffective results similar to those produced when conventional copper molds are used.
Another method of varying applicable thermal conditions involves heating the molten steel surface in the mold, such as by arc heating or the like. One method uses induction-heating by the use of a flat-type coil as disclosed in Japanese Patent Laid-Open No. 56-68565, in which heat input into the meniscus can be controlled independently of the casting conditions. The flat-type coil is placed just above the molten steel surface in the mold so as to apply alternating current, thereby uniformly heating the surface of the molten steel. Since a high frequency current is caused to flow into the heating coil, Joule heat is generated on the conductor, and is likely to damage the coil. Accordingly, cooling water is caused to flow into the coil in order to prevent such damage. However, the presence of a flat-type coil arranged just above the molten steel surface presents serious problems.
(1) In order to obtain good heat efficiency, it is necessary to position the heating coil close to the molten steel surface. However, this raises the level of the molten steel surface and immerse the heating coil in the molten steel, thus damaging the coil and further causing the leakage of cooling water, which contacts the molten steel, resulting in a steam explosion.
(2) In general, a swirl-type level sensor for measuring the level of the molten steel is usually provided just above the molten steel surface. Such a sensor is vulnerable to heating by the heating coil with resulting damage.
(3) The heating coil must be detached from time to time for the exchange of an immersion nozzle and tundish in order to avoid damage of the coil.
(4) Mold powder is normally introduced into the molten steel to enhance the temperature maintenance on the molten steel surface, the absorption of non-metallic inclusions, the lubrication between the mold and the cast products, and the like. The mold powder is continuously supplied from the top in order to ensure the provision of a predetermined volume or more. Since the induction heating coil is thereby subjected to adverse conditions, maintenance control is difficult.